Updated 14 September 2018

Diabetes, cancer link strengthens

People with diabetes may have something else to be concerned about - an increased risk of cancer, according to a report.

Signs of gestational diabetes may be detectable early in pregnancy. . ~

People with diabetes may have something else to be concerned about - an increased risk of cancer, according to a new consensus report produced by experts recruited jointly by the American Cancer Society and the American Diabetes Association.

Diabetes, primarily type 2 diabetes, has been linked to certain cancers, though experts aren't sure if the disease itself leads to the increased risk or if shared risk factors, such as obesity, may be to blame.

Other research has suggested that some diabetes treatments, such as certain insulins, may also be associated with the development of some cancers. But the evidence isn't conclusive, and it's difficult to tease out whether the insulin is responsible for the association or other risk factors associated with diabetes could be the root of the link.

"There have been some epidemiological studies that suggest that individuals who are obese or who have [high levels of insulin] appear to have an increased prevalence of certain malignancies, but it's a complex issue because the association is not true for all cancers," explained Dr. David Harlan, director of the Diabetes Centre of Excellence at the University of Massachusetts Memorial Medical Centre in Worcester, and one of the authors of the consensus report.

"So, there's some smoke to suggest an association - but no clear fire," he added.

'Risk of diabetes complications greater'

As for the possible insulin-and-cancer link, Harlan said that because a weak association was found, it's definitely an area that needs to be pursued further. But, he said, that doesn't mean that anyone should change the way they're managing their diabetes.

"Our greatest concern is that individuals with diabetes might choose not to treat their diabetes with insulin or a particular insulin out of concern for a malignancy. The risk of diabetes complications is a far greater concern," noted Harlan.

"It's like when someone decides to drive across the country because they're afraid to fly. While there is a slight risk of dying in a plane crash, statistically it's far riskier to drive."

The consensus report is published in the CA: A Cancer Journal for Clinicians. The experts found evidence of an association between diabetes and an increased risk of liver, pancreas, endometrial, colon/rectal, breast and bladder cancer. Interestingly, they found evidence that diabetes is associated with a reduced risk of prostate cancer.

"There's a strong consensus that there is a link between diabetes and cancer, and there are some very plausible biologic links," said the report's lead author, Dr Edward Giovannucci, a professor of nutrition and epidemiology at the Harvard School of Public Health in Boston. He said that insulin, and insulin-like growth factors, can promote some cancers, and that many people with type 2 diabetes have high levels of circulating insulin, sometimes for years before they're diagnosed with diabetes.

And, he said, there's definitely an overlap in some of the risk factors for both type 2 diabetes and cancer, especially obesity.

Lifestyle changes key

The panel also found research that suggests the commonly used type 2 diabetes medication, metformin, might offer users some protection against cancer. Giovannucci said this may be because the drug reduces insulin resistance and lowers the need for additional insulin, or that metformin may act on cells in other direct or indirect ways.

Giovannucci said that the most important message to take away from this research is the "profound effects that lifestyle changes can have on your risk of diabetes and your risk of cancer."

He said it's not always the most popular message, but to lower the risk of cancer, it's important to reduce your body weight, exercise, improve your diet and avoid smoking.

Alice Bender, the nutrition communications manager for the American Institute for Cancer Research (AICR), said she wasn't surprised by the findings in the consensus report. "What we're seeing is that there are a lot of commonalities between chronic diseases and their risk factors," she said.

Bender agreed with Giovannucci's suggestions and said the AICR recommends three guidelines for everyone: Maintain a healthy body weight; be physically active for at least 30 minutes a day; and, eat a mostly plant-based diet that's healthy and varied.

"At least for cancer, we know that each factor independently lowers the risk of certain cancers, but all three done together are even more powerful. And, I suspect that's the case for preventing type 2 diabetes also," she said.

Bender also emphasized the need to moderate the consumption of alcohol, which means no more than one drink per day for women and no more than two drinks per day for men. - (Reuters Health, June 2010)

Ask the Expert

CANSA’s purpose is to lead the fight against cancer in South Africa. Its mission is to be the preferred non-profit organisation that enables research, educates the public and provides support to all people affected by cancer. Questions are answered by CANSA’s Head of Health Professor Michael Herbst. For more information, visit cansa.org.za.

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